Chapter 19 Configuring Igmp Snooping And Mvr; Understanding Igmp Snooping - Cisco WS-C3560-48PS-S Software Configuration Manual

Understanding IGMP Snooping

Understanding IGMP Snooping
Layer 2 switches can use IGMP snooping to constrain the flooding of multicast traffic by dynamically
configuring Layer 2 interfaces so that multicast traffic is forwarded to only those interfaces associated
with IP multicast devices. As the name implies, IGMP snooping requires the LAN switch to snoop on
the IGMP transmissions between the host and the router and to keep track of multicast groups and
member ports. When the switch receives an IGMP report from a host for a particular multicast group,
the switch adds the host port number to the forwarding table entry; when it receives an IGMP Leave
Group message from a host, it removes the host port from the table entry. It also periodically deletes
entries if it does not receive IGMP membership reports from the multicast clients.
For more information on IP multicast and IGMP, refer to RFC 1112 and RFC 2236.
Note
The multicast router sends out periodic general queries to all VLANs. All hosts interested in this
multicast traffic send join requests and are added to the forwarding table entry. The switch creates one
entry per VLAN in the IGMP snooping IP multicast forwarding table for each group from which it
receives an IGMP join request.
The Catalyst 3560 switch supports IP multicast group-based bridging, rather than MAC-addressed based
groups. With multicast MAC address-based groups, if an IP address being configured translates (aliases)
to a previously configured MAC address or to any reserved multicast MAC addresses (in the range
224.0.0.xxx), the command fails. Because the Catalyst 3560 switch uses IP multicast groups, there are
no address aliasing issues.
The IP multicast groups learned through IGMP snooping are dynamic. However, you can statically
configure multicast groups by using the ip igmp snooping vlan vlan-id static ip_address interface
interface-id global configuration command. If you specify group membership for a multicast group
address statically, your setting supersedes any automatic manipulation by IGMP snooping. Multicast
group membership lists can consist of both user-defined and IGMP snooping-learned settings.
If a port spanning-tree, a port group, or a VLAN ID change occurs, the IGMP snooping-learned multicast
groups from this port on the VLAN are deleted.
These sections describe characteristics of IGMP snooping on the switch:
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Catalyst 3560 Switch Software Configuration Guide
19-2
IGMP Versions, page 19-3
Joining a Multicast Group, page 19-3
Leaving a Multicast Group, page 19-5
Immediate-Leave Processing, page 19-6
IGMP Report Suppression, page 19-6
Chapter 19 Configuring IGMP Snooping and MVR
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